From owner-chemistry@ccl.net Thu Jan 31 08:12:00 2013
From: "Andreas Klamt klamt/a\cosmologic.de"
To: CCL
Subject: CCL: COSMO vs COSMO-RS
Message-Id: <-48161-130131073524-19610-Po7VVrXiOm/fIOnJYe5r7w^server.ccl.net>
X-Original-From: Andreas Klamt
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Date: Thu, 31 Jan 2013 13:35:15 +0100
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Sent to CCL by: Andreas Klamt [klamt=cosmologic.de]
Dear Sina,
there are several potential reasons for the large difference. On the one
hand you used qute different basis sets for the COSMO calculations at
eps=78 and the eps=infinity calculations. Admittedly I do not expect
that this changes the results too much.
I am not completely sure how you did the COSMO-RS reaction calculation.
Did you use the thermodynamic cycle inserting the gasphase reaction
energy from the b3-lyp and a def2-QZVP calculation? If so, then the
latter result, i.e.
GReaction-COSMO-RS-Water - GReaction-gas= -2.05 Kcal/mol
should be nothing else than the dG_solvation difference of educts and
products on BP-TZVP-COSMO-RS level, since GReaction-gas cancels out.
Can you just check and confirm that?
Please note that bare COSMO (unlike other implicit solvation models) is
not parameterized for giving optimal dG-solvation values. It just trets
the electrostatic component, and not the dispersion, and other
non-electrosttic contributions. Hence you cannot expect good
quantitative results for dG_hydrtion from using bare COSMO. COSMO-RS is
my recommended method to take all these things into account.
Finally it should be noted that the recommended procedure for
dG_reaction calculation requires to include the vibr., rot. and transl.
free energy contributions in the gasphase, which will be very important
for a reaction in which the number of particles changes from 1 to 4.
These can be calculated for exmple with the freeH tool in TURBOMOLE. But
this contribution would appear in the gasphase as well as in the liquid
phase and thus should not influence your differential results.
In summary, I do not find your result too surprising. The more relible
answer should be the COSMO-RS answer, but please note that your
calculations are still suboptimal.
Best regards
Andreas
Am 31.01.2013 00:13, schrieb Sina Behtash behtash!=!email.sc.edu:
> Sent to CCL by: "Sina Behtash" [behtash ~ email.sc.edu]
>
> I am using both COSMO (implemented in Turbomole 6.0) and COSMOtherm/COSMO-RS to calculate the solvent effects on the free energy of reaction for the following reaction:
>
> Glucose --> HMF + 3 H2O
>
> First, I optimized all the reactants and products in the gas phase using b3-lyp and a def2-QZVP basis set. Next, assuming that the geometry does not change significantly in the presence of water, I used the optimized structure to run a COSMO single-point energy calculation with the same basis set and dielectric constant of water at 298 K which is 78.39. According to the COSMO calculation,
>
> GReaction-COSMO - GReaction-gas= -15.2 Kcal/mol
>
> Next, in order to generate relevant input for COSMO-RS calculations, I ran another COSMO single-point energy calculation using the BP functional and TZVP basis set. Then I used COSMOtherm/Reactions and the BP-TZVP data base to run COSMO-RS calculations for my reaction at T=298K.
>
> The result shows that,
>
> GReaction-COSMO-RS-Water - GReaction-gas= -2.05 Kcal/mol
>
> I was expecting that for water, the COSMO and COSMO-RS calculations should give almost the same result. However, in this case the difference in the effect of water on the free energy of reaction is around 13 Kcal/mol. Can anyone explain to me why the difference is so large? Im wondering if the effects of hydrogen bonding and electrostatic interactions can be this significant.
>
> Sina Behtash
> Ph.D. Student
> Department of Chemical Engineering
> University of South Carolina
> Phone: (803)777-0611
> Email: behtash-$-email.sc.edu>
>
>
--
Prof. Dr. Andreas Klamt
CEO / Geschäftsführer
COSMOlogic GmbH & Co. KG
Imbacher Weg 46
D-51379 Leverkusen, Germany
phone +49-2171-731681
fax +49-2171-731689
e-mail klamt^^^cosmologic.de
web www.cosmologic.de
[University address: Inst. of Physical and
Theoretical Chemistry, University of Regensburg]
HRA 20653 Amtsgericht Koeln, GF: Prof. Dr. Andreas Klamt
Komplementaer: COSMOlogic Verwaltungs GmbH
HRB 49501 Amtsgericht Koeln, GF: Prof. Dr. Andreas Klamt